Download Bipolar Junction Transisitors-III - CIIT Virtual Campus: Digital Library

COMSATS Institute of Information Technology
Virtual campus
Islamabad
Dr. Nasim Zafar
Electronics 1
EEE 231 – BS Electrical Engineering
Fall Semester – 2012
BJT-Transistor Characteristics
and Parameters:
Lecture No: 15
Contents:
 Transistor Characteristics and Parameters.
 The Gain Factors: DC Beta( ) and DC Alpha ( ).
 Relationship of  and  .
 Early Effect.
 Maximum Transistor Ratings.
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References:
 Microelectronic Circuits:
Adel S. Sedra and Kenneth C. Smith.
 Electronic Devices :
Thomas L. Floyd ( Prentice Hall ).
 Integrated Electronics:
Jacob Millman and Christos Halkias (McGraw-Hill).
 Electronic Devices and Circuit Theory:
Robert Boylestad & Louis Nashelsky ( Prentice Hall ).

Introductory Electronic Devices and Circuits:
Robert T. Paynter.
Reference:
Chapter 4 – Bipolar Junction Transistors:
Figures are redrawn (with some modifications) from
Electronic Devices
By
Thomas L. Floyd
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Bipolar Junction Transistors
BJTs-Circuits
C
B
E
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Transistor Types:
 MOS - Metal Oxide Semiconductor
 FET - Field Effect Transistor
BJT - Bipolar Junction Transistor
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◄
Transistor Characteristics
and
Hybrid Parameters
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An Overview of Bipolar Transistors:
 While control in an FET is due to an electric field.
 Control in a bipolar transistor is generally considered to be due
to an electric current.
– current into one terminal
determines the current
between two others
– as with an FET, a
bipolar transistor
can be used as a
‘control device’
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Transistor Characteristics:

Transistor Geometry.

Carrier motion (mobility).

Collector “collection efficiency” (Alpha).

Asymmetry: Efficiency / Breakdown voltages.

NPN transistors are normally better than PNP since electron
mobility is better than hole mobility.
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Transistor Biasing Configurations
and Operation Modes:
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Transistor Biasing Configurations:
1. Common-Base Configuration (CB) :
input = VEB & IE ; output = VCB & IC
2. Common-Emitter Configuration (CE):
input = VBE & IB ; output = VCE & IC
3. Common-Collector Configuration (CC):
input = VBC & IB ; output = VEC & IE
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Modes of BJT Operation:
IC(mA)
Saturation Region
IB = 200 mA
30
Active Region
IB = 150 mA
22.5
IB = 100 mA
15
IB = 50 mA
7.5
Cutoff Region
IB = 0
0
VCE (V)
0
5
10
15
20
 Active: BJT acts like an amplifier (most common use).
 Saturation: BJT acts like a short circuit.
Cutoff: BJT acts like an open circuit.
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Modes of BJT Operation:
 Active Region: Region where current curves are practically flat.
•
•
In Active Region, the transistor is on. The collector current is proportional
to and controlled by the base current IC (IC = βIB) and relatively insensitive
to VCE.
In this region the transistor can be used as an amplifier.
 Cutoff Region: Current reduced to zero.
– The transistor is off. There is no conduction between the collector and
the emitter. (IB = 0 therefore IC = 0).
– Equivalent to an off-state and the transistor behaves like an open
switch. Low current flow, High Voltage.
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Modes of BJT Operation:
 Saturation Region:
– In Saturation region: The transistor is on. The collector current varies
very little with a change in the base current in the saturation region.
– The output voltage VCE is small, a few tenths of a volt.
– The collector current is strongly dependent on VCE unlike in the active
region.
– Ideal transistor behaves like a closed switch.
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Modes of BJT Operation:
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Transistor Characteristics
and
Hybrid Parameters
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1. DC-Current Gain Parameters:
DC Beta (dc) and DC Alpha (dc ):
Two quantities of great importance in the characterization of
the transistors are:
 common-base current gain  .
 common-emitter current gain .
 = Common-emitter current gain
 = Common-base current gain
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DC Common-Emitter Current Gain :
 Current gain β, usually designated as an equivalent
hybrid (h) parameter hFE, is defined by:
hFE = DC
 The ratio of the dc collector current IC to the dc base current
IB is defined as the dc gain factor Beta (dc) of a transistor.
Thus:
 = IC/IB
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DC Common-Emitter Current Gain :
 = Common-emitter-current gain (typical 50-200)
iC

iB
VBE
iC  I S e
iB 
IS

VBE
e
VT
VT
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DC Common-Base Current Gain  :
 Current gain  , is also referred to as hFB and is defined
by:
hFB = DC
 The ratio of the dc collector current IC to IE, due to the
majority carriers, are related by a quantity called dc Alpha (dc ):
=
Also:
IC / IE
IC  βIB
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DC Common-Base Current Gain  :
 = Common-Base Current Gain (typical 0.99)
iC

iE
VBE
iC  I S e
iE 
IS

VT
VBE
e
VT
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Beta () or Amplification Factor:
 IC and IB are determined at a particular
operating point, Q-point (quiescent point).
 Typical values of dc range from:
30 < dc < 200  2N3904
 On data sheet, dc= hFE with h is derived from ac hybrid
equivalent circuit. hFE are derived from forward-current
amplification and common-emitter configuration
respectively.
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AC Common-Base Current Gain  :
 For ac situations, where the point of operation moves
on the characteristics curve, an ac alpha is defined by:
  IC
IE
 Alpha, a common base current gain factor, gives the
efficiency of the transistor for a current flow from the
emitter to the collector.
 The value of  is typical from 0.95 ~ 0.99.
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2. Relationship of DC and DC:
2. Relationship of DC and DC:
 = Common-base current gain (0.95-0.99)
 = Common-emitter current gain (typical 50-200)
 The relationship between the two parameters are:


 1


1
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3. Performance Parameters for PNP:
Emitter Efficiency:
 
I EP
I
 EP
I EP  I EN
IE
Fraction of emitter current carried by holes.
We want  close to 1.
Base Transport Factor:
IC
αT 
I Ep
Fraction of holes collected by the collector.
We want T close to 1.
Common Base dc Current Gain:
I C  T I EP  T I E  dc I E
 dc   T 
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The Early Effect (Early Voltage)
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Early Effect (base width modulation):
 In a Common Emitter Configuration, IC depends on VCE.
 An increase in VCE means that the CB junction becomes more
reverse biased.
 The depletion layer width increases into the base, reducing the
effective base width.
 Hence the base transport efficiency (α) and β increase with
increasing VCE.
 This effect is known as base width modulation or the Early
Effect.
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The Early Effect (Early Voltage)
IC
IB
-VA
Common-Emitter Configuration
VCE
Green = Ideal IC
Orange = Actual IC (IC’)
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Actual Output Characteristics
 Salient features are:
 The finite slope of the plots (IC depends on VCE).
 A limit on the power that can be dissipated.
 The curves are not equally spaced (i.e β varies with base
current, IB).
Note: The finite slope of the (IC-VCE) plot would manifest
itself as an output resistance. This would appear in a more
detailed a.c. equivalent circuit of the transistor than the
one we shall derive from the ideal curve.
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Output Characteristics:
Ideal C-E Output Characteristics:
Actual C-E Output Characteristics:
IB =
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an Example-The Early Effect:
• Given:
• Find:
The common-emitter circuit below with
IB = 25mA, VCC = 15V,  = 100 and VA = 80.
a) The ideal collector current
b) The actual collector current
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Power Across BJT:
 PBJT = VCE * iCE
 Should be below the rated transistor power.
 Should be kept in mind when considering heat
dissipation.
 Reducing power increases efficiency.
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Derating PDmax
 PDmax is usually specified at 25°C.
 The higher temperature goes, the less is Pdmax
 Example:
– A derating factor of 2mW/°C indicates the power
dissipation is reduced 2mW each degree centigrade
increase of temperature.
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Summary of Bipolar Transistors:
 Bipolar transistors have three terminals:
collector, base and emitter.
 The base is the control input.
 Two polarities of device: npn and pnp
 The collector current is controlled by the base
voltage/current IC = hFEIB.
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Summary of Bipolar Transistors:
 Bipolar transistors are widely used in both analogue and digital
circuits.
 They can be considered as either voltage-controlled or currentcontrolled devices.
 Their characteristics may be described by their gain or by their
transconductance.
 The majority of circuits use transistors in a common-emitter
configuration where the input is applied to the base and the output is
taken from the collector
 Common-collector circuits make good buffer amplifiers
 Bipolar transistors are used in a wide range of applications
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